Vladislav Yur'EVich Zaslavsky

At present, the novel compact fusion reactors with strong magnetic fields, including the DEMO project, require the development of 220–240 GHz megawatt continuous-wave heating sources. In this paper, gyrotrons of planar geometry of interaction space with a sheet polyhelical electron beam and transverse energy extraction are considered as such sources. An advantage of this design in comparison with the conventional cylindrical gyrotron configuration is the possibility to ensure effective mode selection over the open transverse coordinate in combination with radiation outcoupling. The theoretical analysis and numerical 3D PIC simulations of a 230 GHz megawatt power planar gyrotron operating at the first harmonic of the cyclotron frequency are performed. The paper considers the feasibility of single-mode generation with output power exceeding 1 MW, efficiency of 30%, and Ohmic loads less than 1.5 kW/cm2, which is compatible with the continuous-wave operation regime.

We propose a gyrotron scheme with a two-mirror echelette-type resonator to obtain high-power continuous wave terahertz (THz) radiation, in which a corrugated profile applied to one of the mirrors is specified in the transverse plane (with respect to an electron's translation velocity). The gyrotron has been driven by a sheet helical electron beam (2 A/20 keV) with a pitch-factor of 1.2 and acceptable velocity spread formed by a planar magnetron-injection gun. The theoretical analysis was carried out both within the framework of a self-consistent theory and on the basis of 3D particle-in-cell simulations. The linear and nonlinear stages of electron–wave interaction were analyzed. Simulations demonstrate the operability of the proposed gyrotron circuit in the frequency range of 0.79 THz, operating at the second cyclotron harmonic with a magnetic field strength of 15 T at the high-order TE0,50,1 mode. The output power level in the steady-state single-mode oscillation regime reached several kilowatts with an Ohmic loss value of less than 20%. Proof-of-principle experiment was carried out to study the selective characteristics of a two-mirror echelette-type resonator in the G-band, demonstrating the feasibility of the proposed gyrotron design.

A design for a planar relativistic surface wave generator operating in the G-band has been developed through theoretical analysis and three-dimensional particle-in-cell modeling. The device is based on a sheet electron beam produced by the SINUKI accelerator (600 keV/1 kA/17 ns, Institute of Applied Physics, Russian Academy of Sciences) using explosive electron emission. The electrodynamic system of the generator employs a two-dimensionally periodic slow-wave structure that implements a two-dimensional distributed feedback mechanism and enables mode selection across the transverse spatial coordinates. The simulation demonstrates the feasibility of stable single-mode generation in this configuration, achieving an efficiency of approximately 8% with an oversize factor (defined as the ratio of system width to wavelength) of about 15. Experimental results confirm a stable single-mode generation regime at 160 GHz, with pulse durations of up to 5 ns and output power reaching approximately 30 MW.

The project of a powerful spatially-extended W-band Cherenkov maser of planar geometry is currently being developed in collaboration between the BINP RAS and IAP RAS. The experimental basis of the project is the «ELMI» accelerator complex forming the electron beams with parameters of 1 MeV / 3 - 5 kA / 3 μs and a cross-section of 0.4 cm × 17 cm (sheet configuration). A key component of the electrodynamic system of the generator is a slow-wave structure with two-dimensional periodic corrugation, which provides a two-dimensional distributed feedback mechanism. The paper presents the simulation results of the developed generator and data from the first series of experiments on its realization, in which the excitation of the orotron-type modes was observed. Approaches to the suppression of the parasitic modes and the implementation of the surface wave generation regime with the prospect of the generated radiation power enhancement are discussed.

Comparative theoretical and experimental studies of G-band surface-wave oscillators (SWOs) with one-dimensional (1D) and two-dimensional (2D) planar slow-wave structures (SWSs) has been carried out. These oscillators have been driven by a sheet electron beam with a cross-section of 0.3 mm × 20 mm formed by the SINUKI accelerator 650 keV / 1 kA / 17 ns. At the oversize factor of ~ 15 (the ratio of the transverse size to the radiation wavelength), in the case of 1D SWS, a multistable generation with excitation of various transverse modes was observed, characterized by a jitter of the radiation frequency. In contrast, 2D SWS ensures a single-mode generation regime stable to variations of beam parameters. In accordance with the simulations, narrowband generation with excitation of the fundamental mode of 2D SWS was observed in the experiment at an operating frequency of 160 GHz with an output power of 30 MW and an electron efficiency of 5%.

A three-dimensional (3D) numerical simulation of scattering of seismic surface Rayleigh and longitudinal waves propagating through the ground, the density and elasticity of which are typical of geomedium. At the soil boundary, there is local unevenness in the form of a hollow hemispherical notch (a truncated sphere). The dependence of the direction of the scattering field on the type of inhomogeneity is shown. From literature it is known that in the transition to another type of heterogeneity, for instance, to covering the boundary with a thin inert (massive) layer in the form of a circle, the forward scattering occurs. A pulsed mode of inhomogeneity probing is considered. As an emitter, it is proposed to use a short pulse source, e.g., a hydroacoustic emitter, or something similar - a pulsating monopole, shallowly immersed under the free boundary. This leads to the generation of such elastic waves as the surface Rayleigh and back-reflected longitudinal waves, which are usually recorded using an array of seismic receivers installed on the free boundary according to the grid pattern. The spatial amplitude distribution of the wave field is analyzed in the vertical (at the center of the inhomogeneity) and horizontal (at the free boundary) sections of the medium. The characteristic features of the wave field are caused by the influence of its scattering on the local inhomogeneity. The features in the image of wave reliefs that arise at the intersection of the wave fronts of longitudinal waves - reflected from the free boundary and scattered on the local inhomogeneity - are studied. Informative signs, indicating the presence of the local heterogeneity and enable diagnostics of its parameters are established. The ways to improve the validity and reliability of algorithms for detection and classification of inhomogeneities and for evaluation of their difficulties using the listed types of waves are discussed. Based on the use of increasingly shorter probing pulses, the possibility of a detailed representation of reliefs and, consequently, the potentially achievable spatial resolution in probing the local subsurface inhomogeneities are demonstrated.

We develop a concept of a high-power dual-frequency CW radiation source based on a gyrotron with two co-axis electron beams which independently excite respective waveguide modes at the $1^{\text {st}}$ and $2^{\text {nd}}$ cyclotron harmonics. For this purpose, the operating modes and electron injection radii should be selected in such a way that each beam has a maximum coupling with one of the modes and is practically not coupled with the other. In a moderately oversized resonator, the specified condition can be satisfied, for example, for the whispering-gallery mode TE $_{{5},{1}}$ and the bulk mode TE $_{{4},{3}}$ . Simulations based on both an averaged multimode model and a 3D PIC (particle-in-cell) code show that when these modes are excited by 1-A/30-kV electron beams, it is possible to provide simultaneous multi-kW generation at frequencies of 14 and 28 GHz. The proposed type of radiation sources looks quite promising for various plasma applications.

In this paper, we studied the operability of various components of vacuum electronic devices manufactured using the novel chemical metallization of photopolymer 3D-printed structures technology (CMPS), which is being applied at the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS), for operation from microwave to sub-terahertz ranges. The key feature of this production method is the 3D printing (SLA/DLP, MJM technologies) of products and their further metallization. The paper presents the main stages of the process of chemical copper plating of polymer bases in various electrodynamic systems with complex shapes. A significant difference in the geometry and operating conditions of the created elements forms certain approaches to their production, as described in this work. Experimental studies of the implemented microwave components were carried out up to 700 GHz in the “cold” measurements; some electrodynamic structures were examined under conditions of sub-gigawatt peak power; and complex-shaped electrodes with cooling channels were tested under a continuous high thermal load. The conducted research has demonstrated the high potential of the developed methods of additive manufacturing of microwave device components and the prospects for their successful application in the described areas.

A novel concept of a frequency-tuned sub-terahertz gyrotron based on a combination of an irregular low-frequency resonator and an external reflector has been proposed recently. A simulation was carried out for a fundamental-cyclotron-harmonic gyrotron that demonstrates the possibility of achieving high (10–30%) efficiencies in a wide (~10%) frequency range. A possible solution to the problem of narrow-band frequency-tunable external reflectors in the form of so-called modified planar Bragg structures is discussed. The manufacturing of such structures on the basis of a novel additive technology based on photopolymer 3D printing, as well as the results of “cold” experiments of the manufactured samples, are described in the paper.